The present invention relates to creation of a stress wave for causing material damage or destruction, particularly relevant to intracorporeal shock wave lithotripsy, particularly to the use of acoustic energy for lithotripsy, and more particularly to a device having a transducer tip for converting optical (laser) energy into acoustic energy to produce stress waves for local and targeted material destruction, primarily for medical applications such as lithotripsy.
The standard in the non-surgical treatment of biliary and urinary calculi, more commonly known as gallstones and kidney stones, is extracorporeal shock wave lithotripsy (ESWL). The procedure entails placing the patient in a water bath that holds an acoustic generator. The acoustic wave is focused through the water, usually with an ellipsoid reflector, and into the patient so that the focal point is at the location of the calculi. As the acoustic wave is focused it develops into a shock-like waveform. Several different commercial lithotripters are available, generating compressive pressures up to 120 MPa and tensile pressures of 10 MPa within a focal region ranging from about 0.2 cm3 to 16 cm3. Repeated pulses of the shockwave, as many as 2,000, break apart the calculi. The mechanism is not fully understood, but is most likely due to either cavitation near the stone surface or the development of stress and shear within the stone itself.
Ideally, ESWL would exclusively target the stones, under ultrasound or fluoroscopic guidance, without affecting the soft tissue near the focal point or the tissue lying in the path of the shockwave. In practice, however, complications arise that are related to the non-exclusive application of the shockwave. These complications include bleeding in almost all cases, common kidney failure, pancreatitis, bruise-like skin lesions at the site through which the shockwave passes, and in rare cases broken bones and heart arrhythmias3. Reducing the shockwave intensity to limit these complications reduces the efficiency of the devices to destroy stones. ESWL can not be used for patients (i) with predisposition to complications, (ii) obese patients or pediatric patients because the focal distance of the lithotripter cannot be adjusted to the site of the stone, or (iii) pregnant patients. In addition, ESWL fails to destroy some stones due to stone composition or mobility. Alternate techniques for removal of the stones must then be used.
The two most utilized alternatives are open surgical procedures and intracorporeal laser lithotripsy. The surgical technique is generally considered a last resort as it is costly and requires general anesthesia with hospital stays of up to a week, whereas ESWL is generally an outpatient procedure. Laser lithotripsy entails the transurethral guidance of an optical fiber through a flexible endoscope to the position of the stone. To create a radically expanding shock wave with a high enough intensity to break apart the calculi, the laser radiation must be of relatively high intensity. This high intensity beam can damage soft tissue. Successful calculi destruction is accomplished in a high percentage of patients, but damage to the bladder or ureter is relatively common, with perforations in about 6% of the cases.
The present invention utilizes laser energy directed through an optical fiber, as in laser lithotripsy, but additionally incorporates a shaped optical fiber tip, a transducer tip, or endpiece that converts optical (laser) energy into acoustic energy. The acoustic energy (stress waves) destroys targeted material with minimal damage to the surrounding (untargeted) tissue. The transducer tip or endpiece includes a material, such as an exogenous absorbing dye and an acoustic lens. Whether radiation absorption takes place in a transducer tip or in the ambient media, the generated stress waves travel through the transducer or endpiece and are focused by the acoustic lens to a spot outside the endpiece. The endpiece also serves to confine the laser radiation to eliminate the danger of direct absorption in soft tissue and subsequent perforation. Acoustic energy has little effect on soft tissue. The focusing of the acoustic wave will allow for both specific targeting of the shock wave to the stone and the generation of pressures equivalent to ESWL with a lower-cost laser than what is currently required for laser lithotripsy.
It is an object of the present invention to provide a more effective means for mechanically disrupting or damaging biological or non-biological materials.
A further object of the present invention is to provide a more effective lithotripsy procedure.
A further object of the invention is to provide a lithotripsy procedure that utilizes acoustic waves produced by laser radiation.
A further object of the invention is to provide a device for lithotripsy that utilizes optical energy to produce acoustic waves for the destruction of targeted material.
Another object of the invention is to utilize laser energy to produce acoustic waves for lithotripsy procedures.
Another object of the invention is to provide a lithotripsy technique that involves depositing laser radiation into an exogenous absorber contained within a capsule at the end of an optical fiber, which produces an acoustic wave that can be focused by an acoustic lens onto a target tissue or other material causing the destruction thereof.
Another object of the invention is to provide a device for lithotripsy that can convert compressive stress waves into a tensile stress wave for more effectively disrupting material.
Another object of the invention is to provide a device for lithotripsy that includes an acoustic lens such that the transmitted acoustic wave is shaped or focused.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The present invention incorporates the use of a laser with a transducer tip that converts optical energy into acoustic energy to produce stress waves for local and targeted material destruction, primarily for medical applications, such as shock wave lithotripsy. Energy from a laser, such as a q-switched Nd:YAG laser, is directed into an optical fiber. A transducer tip is attached to the distal end of the optical fiber, and encapsulates an exogenous absorbing dye, for example. Under proper irradiation conditions (high absorbed energy density and short pulse duration) a significant stress wave is produced via thermoelastic expansion of the absorber and the stress wave is directed onto a targeted material for destruction thereof. Further, the transducer tip can be formed into an acoustic lens such that the transmitted acoustic wave is shaped or focused. In addition, the transducer tip can be constructed such that compressive stress waves can be inverted into a tensile stress wave. Tensile stresses may be more effective in disrupting material as most materials are weaker in tension than compression. Based on theoretical estimates, the stress amplitudes produced by this invention can be magnified more than 100 times, greatly improving the efficiency over that of an unfocused opto-acoustic transducer.